Optical setup helps researchers hide an event from time

Researchers have managed to perform a bit of optical trickery to ensure that, …

Cloaking devices are one of the inventions of science fiction that have made a few tentative steps towards the real world in recent years. Now, researchers have moved the concept into the fourth dimension, creating a setup that hides a specific point in time from being perceived by observers. But if you want to make an event disappear, you have to act fast: right now, we can only hide a few picoseconds worth of time.

The cloaking devices we've made all work based on a similar principle: light that enters the device is bent in such a way that when it exits, its location and direction make it appear that the device itself, and anything within it, were not present. In other words, while within the device, light travels as if it were present. It's just that, once it exits the other side, there's no evidence that anything unusual has taken place. The same general idea governs the action of a temporal cloaking device.

The basic idea is that, when it's not in operation, a light beam can pass through the cloaking device unhindered. When it's switched on, a short temporal gap is opened up in the beam, then sealed back up on its way out of the hardware. One way to think of this is to view the light beam as a bit of old-fashioned magnetic tape. You can cut the tape so that a single instant of a recording can be physically separated. While separated, you can pass anything you want through the gap, but when you glue the tape back together, the recording is seamless. There's only a before and after while the tape is cut and separated.

It's easy to do that with tape, but a bit harder to do it with a beam of light. The key to the process is what's being termed a split time lens, which is matched with a dispersive medium. When activated, the lens takes the light that comes before the point of cloaking and shifts it to bluer wavelengths, which travel faster through the dispersive medium than the base speed of the light in the same medium. At the cloak point, the lens switches, shifting the light beam to longer, redder wavelengths. These travel through the dispersive medium more slowly.

The end result is that a gap develops in the beam of light as the blue light races ahead and the red light slows down. In this cloaking device, that gap maxes out at about 50 picoseconds, long enough for a very brief event to occur (in this case, the event was simply an interaction with a pulse from another laser). The light can then be sent through a dispersive medium with the opposite effect, slowing the bluish light back down while accelerating the red.

By the time it exits the device, the gap has been sealed back up and the effects of the wavelength shifts reversed. Any events that took place during the 50 picosecond gap (as long as they occur in both the right place and time) never happened, at least as far as the light beam that exits is concerned.

The authors timed things so that a picosecond laser pulse should interact with the light beam at the right time and place. When the cloaking device was off, a clear signal was apparent in the output. Once it was switched on, however, the signal dropped to background levels. By sampling many events, it was still possible to pick out a signal with the expected frequency, but it was over ten times smaller than the one present when the cloaking device was switched off.

Fifty picoseconds isn't a lot to work with. With the device as it now stands, the authors think they can stretch out the temporal gap to about 110 picoseconds; changing the optics a bit would push it up against the limits of optical cabling, which maxes out at about 50km. Using the full length of the cables would net you a gap of a few nanoseconds.

As with the spatial cloaking devices, it's not obvious that the temporal cloaking device will ever have any sort of practical utility. Still, it's an impressive display of what we can do with a bit of finely tuned optics.

If the article noted more explicitly the fact that the base speed of light in a medium is less than the speed of light in a vacuum, it would be clearer that light traveling faster than the base speed of light in the medium does not imply that anything is travelling faster than the speed of light.

If the article noted more explicitly the fact that the base speed of light in a medium is less than the speed of light in a vacuum, it would be clearer that light traveling faster than the base speed of light in the medium does not imply that anything is travelling faster than the speed of light.

They were explicit that they weren't talking about the speed of light in a vacuum. That should be enough for readers, especially since the headline and article would likely be different and a bit more sensational if your inference was the case.

"When activated, the lens takes the light that comes before the point of cloaking and shifts it to bluer wavelengths, which travel faster through the **dispersive medium** than the base speed of the light in the **same medium**."

Am I the only one who does not get how this is a "time cloaking device"?

The way I read it, if the light was a river of water (with a start and end, let's say there's a dam), it sounds like they split the river, sent half of it around a slower path, drove a car across while each section was dry, then said they cloaked time? Is that about right?

From this description it sounds like we could equally say that magicians have managed to "cloak matter" when they make a rabbit disappear and reappear. Technically true, but a little misleading...

Time = change so if something changes then you have time. "At least as far as the light beam that exits is concerned" means there's no "hiding" from time, if that could somehow make sense in the first place.

Am I the only one who does not get how this is a "time cloaking device"?

The way I read it, if the light was a river of water (with a start and end, let's say there's a dam), it sounds like they split the river, sent half of it around a slower path, drove a car across while each section was dry, then said they cloaked time? Is that about right?

From this description it sounds like we could equally say that magicians have managed to "cloak matter" when they make a rabbit disappear and reappear. Technically true, but a little misleading...

It's not cloaking time, it's using differentials in the speed of different wavelenghts of light to hide events. In your example, an observer wouldn't be able to see the car drive across the river, only an uninterrupted flow (as I understood the article).

So, as a layperson, perhaps someone more educated in this field can help me:

Could you not put this on a loop, so that while it only works for 50-110 picoseconds, this "event" is repeated very rapidly, thereby essentially extending the effect by stringing together a number of micro "events" together with (little to no) gap between each?

Please use monosyllabic words in your response for this caveman. Grunt.

On a related note, I don't see what the big fuss is about the Voyager program. Essentially, all they did was make a slightly fancier paper airplane, which we've been doing for decades if not centuries.

So, as a layperson, perhaps someone more educated in this field can help me:

Could you not put this on a loop, so that while it only works for 50-110 picoseconds, this "event" is repeated very rapidly, thereby essentially extending the effect by stringing together a number of micro "events" together with (little to no) gap between each?

Please use monosyllabic words in your response for this caveman. Grunt.

No, it just says that the light goes fast for a big length to fix the lag of the big length.(Did that count? I think it does.)

(The disruption still takes the same amount of time as the light normally would if were unhindered, which makes it hard to detect any modifications or readings of the light.)

As I understand it, this must be the first experimental implementation of the space-time cloak stuff that was theoretically laid out a couple years ago as an extension to the the more well known cloak mentioned at the beginning of the article. Although, of course, we are a very long way from being able to mask life size visible events, the technology could prove quite useful in telecoms, where you can mask signals from each-other in certain kinds of junctions.

As with the spatial cloaking devices, it's not obvious that the temporal cloaking device will ever have any sort of practical utility.

Then what's the point? These sort of articles are always missing CONTEXT.

Why were researchers doing this? What were they trying to better understand? Was what happened the expected result/outcome, or an interesting side effect / tangent of some other line of research?

Myopic science "articles" like this on Ars drive me nuts, and you guys do it all the time -- make an effort to put things in perspective, and explain in the context of a big picture view, please! Doesn't have to be anything elaborate, but at least bookend the article with a couple paragraphs that explains why a lay-person might give a shit, or find it compelling in any way beyond "well golly gee, how bout that science!"

Makes me wonder if some scientists sit in the lab for a while thinking about the best way to spin the results to make them seem cool. There isn't anything more temporally cloaking about this than cars in a demo derby 'cloaking' themselves to avoid collisions at the cross while maintaining a constant average speed. But, hey, got me and a bunch of others to click. Typically I enjoy the science articles a lot.

Paint it pink + S.E.P. field = win. Alternatively, maybe one could use it to cloak one's departure from the bistro, skipping out on the simulated check but avoiding the whole ugly scene with the simulated angry waiter.

Seriously though, this:

Lone Shepherd wrote:

Quote:

As with the spatial cloaking devices, it's not obvious that the temporal cloaking device will ever have any sort of practical utility.

Then what's the point? These sort of articles are always missing CONTEXT. Why were researchers doing this? [etc...]

How's this for a real world example without any pico-whatsits. Two roads cross at right angles, but without a stop sign / traffic light / etc. There's a steady stream of cars zipping down the north-south road, and every 10 seconds a car zipping across the east-west road, often colliding with a north-south car. We select the proper point in the north-south stream and have a few cars ahead of that point speed up a bit, while a few cars behind that point slow down, opening a gap in the north-south line at just the right moment for the east-west car to nip through the line unscathed. Now the cars that increased speed slow back down, and the cars that dropped back speed up again, closing the gap. Traffic cameras a mile north and south of the intersection detect no change in the flow of north-south traffic.

I see the opposite way, from the perspective of the frog. He starts off by the road at the beginning and makes it across the river unscathed. But something crazy happened when he crossed one of the lane. He gets run over by a car, but doesn't die. Like the collision detection code glitched for a second, or a frame dropped, but somehow the game thought he was still alive and let him continue to the end.

Or maybe one of the cars dumped a bucket of red paint on the frog as he crossed a lane, but he still looks green at the end.

Somehow, some event in time became invisible to outside observers. Or then again, I could be totally off....